This invention relates to apparatus for detecting a malfunction condition in a cathode ray tube (CRT), such as the video picture tube in a television receiver, and more particularly, to apparatus for preventing the possible generation of an excessively high voltage in the high voltage circuit of such a CRT in the event a malfunction occurs.
In a television receiver, the high voltage that must be supplied to the anode of the CRT, known as the anode voltage, typically is produced in a high voltage circuit that is driven from the horizontal deflection circuit normally used for deflecting the CRT electron beam. The horizontal deflection circuit is supplied with pulses synchronized with the horizontal scanning pulses to supply a sawtooth current through the horizontal deflection coils. A retrace period is provided at the end of each sawtooth current waveform for returning the electron beam to its initial scanning position. As is conventional, the retrace period is much shorter than the trace period, i.e., the period during which the electron beam is deflected to scan across the CRT display screen, so that a relatively narrow retrace, or flyback, pulse is produced.
A typical horizontal deflection circuit capable of producing the trace and retrace signals, respectively, is formed of an electronic switch that is driven by the horizontal scanning pulses, this switch being connected to a resonant LC circuit formed of a horizontal output transformer and a resonant capacitor. While the repetition frequency and period of the trace signal is determined by the horizontal scanning pulses, produced in general by a horizontal oscillator, the period of the retrace signal is determined by the resonant frequency of the aforementioned LC circuit. Thus, when the electronic switch is turned ON, a sawtooth-shaped current flows through the horizontal output transformer, and when the electronic switch is turned OFF, the energy then stored in the transformer tends to oscillate at the resonant frequency determined by the LC circuit. A damper diode normally is included in the horizontal deflection circuit so as to permit the retrace current to oscillate for only one-half of the resonant frequency cycle. At the conclusion of this one-half cycle, that is, at the end of the retrace period, current flows through the damper diode and the horizontal output transformer to initiate the next trace period.
The retrace current through the horizontal output transformer, known also as the flyback transformer, that is, the current flowing during the retrace period, produces a voltage pulse across the flyback transformer. This voltage pulse is transformer-coupled to a secondary winding, resulting in a flyback pulse of very high amplitude. This high-amplitude flyback pulse is rectified by a high voltage rectifying circuit and supplied as the anode voltage for the CRT. Typically, this anode voltage is on the order of 25 kV in, for example, a color cathode ray tube. If this anode voltage becomes excessively high, for example, if it is greater than 30-35 kV, injurious X-ray radiation may be generated from the television receiver. Another potential hazard is that such an excessively high voltage may cause a fire.
As described in aforementioned copending Application Ser. No. 688,899, one source of such an excessive high anode voltage resides in the malfunction of the horizontal deflection circuit. For example, if the normal connection between the resonant capacitor and the horizontal output transformer is open-circuited, that is, if this capacitor is disconnected or otherwise damaged to present such an open circuit, then the LC resonant frequency that is determinative of the retrace period will change. In particular, the effective capacitance now connected to the horizontal output transformer no longer is the capacitance of the resonant capacitor; rather, it now is the stray capacitance of the transformer. Since the value of this stray capacitance is less than that of the resonant capacitor, the LC resonant frequency will increase to a very high value. Consequently, the flyback pulse produced across the flyback transformer will contain correspondingly high frequency components which appear as voltage spikes of extremely high magnitude that are rectified by the high voltage rectifying circuit to produce the excessively high anode voltage. A solution to this problem of an excessive anode voltage caused by the opencircuit, or disconnection, of the resonant capacitor is disclosed in the aforementioned copending application.
However, it now has been found that there is yet another cause of such an excessively high anode voltage. In a typical CRT, the horizontal deflection circuit (as well as other circuits) is supplied with an operating voltage by a relatively low voltage power supply that generally is independent of the high voltage circuit used to generate the anode voltage. This operating voltage, which may be on the order of +130 volts, is produced by rectifying the AC power line voltage and then regulating this rectified voltage to produce the DC operating voltage. If, because of some malfunction in the power supply, this operating voltage is increased, the amplitude of the flyback pulse also will increase, resulting in a correspondingly higher anode voltage. While the high voltage protection circuit disclosed in copending application Ser. No. 688,899 will prevent an excessively high anode voltage from being generated in the event of a malfunction in the horizontal deflection circuit, it does not function to prevent an excessively high voltage from being produced in the event of a power supply malfunction.
It had been thought that a high voltage protection circuit could proceed upon the principle of detecting the flyback pulse amplitude and to suitably control the generation of the flyback pulse in the event that such amplitude exceeds a predetermined level. Although this could function to prevent the generation of an excessively high voltage in the event of a horizontal deflection circuit malfunction or a power supply malfunction, such a flyback pulse level detector necessarily would have to withstand the very high flyback pulse voltage levels. Although semiconductor devices, such as power transistors, are known to have this capability, a power transistor is very expensive and use thereof requires a large amount of space to dissipate the heat generated therein. These qualifications render the use of a power transistor less than advantageous.